THE PHYLUM PLATYHELMINTHES 211 



at the time of ejaculation activate the sperm, which begin to undulate. 

 The mating process takes only a few minutes. 



After mating the active sperm migrate from the copulatory sac 

 through the ovovitelline ducts to the seminal receptacles, a pair of 

 cavities next to the pair of ovaries. Mature eggs cross through a par- 

 tition between the ovary and the receptacle, are fertilized, and then 

 pass down the ovovitelline duct together with a group of yolk-packed 

 cells from the vitelline glands. Several eggs are produced at one time. 

 They gather with the yolk cells in the reproductive atrium, where se- 

 cretions from the yolk cells form a membranous capsule surrounding 

 them. As the capsule is released through the genital pore, it is covered 

 with an adhesive secretion from cement glands. A portion of this secre- 

 tion is drawn out into a stalk, which attaches the capsules to the under 

 side of stones and other objects. The eggs develop into embryos that 

 consume the yolk cells in the capsule, and emerge in two or three weeks 

 as miniature flatworms similar to adults. 



77. Dugesia: Regeneration and Polarity 



Many flatworms (but not all) have marked powers of regeneration. 

 These are especially good in Dugesia and in other genera that repro- 

 duce asexually. Cutting a Dugesia in two is, after all, little different 

 from its natural form of division. If the worm is cut across, both pieces 

 will survive and can regenerate a complete worm providing the cut falls 

 somewhere between a line behind the brain and a line a similar dis- 

 tance from the posterior end. In fact, any piece of the worm that is 

 about the size of the head can regenerate a complete worm. Successful 

 regeneration depends upon the regeneration of a head; if this fails to 

 appear, the rest of the body also fails to develop normal proportions and 

 spatial arrangements. 



A particular aspect of flatworm regeneration that has been studied 

 extensively is polarity. Polarity is a general phenomenon in organisms 

 whereby the axes of symmetry tend to be established and maintained. 

 The flatworms are used here as a convenient example in which a con- 

 siderable amount of work has been done. Most of the experiments are 

 concerned with the anteroposterior axis. C. M. Child, working with 

 Dugesia (Fig. 11.9), found that, in general, pieces taken from the middle 

 of a worm regenerate heads at the original anterior ends and tails at the 

 original posterior ends. A more subtle expression of polarity is found 

 in the ease with which the ends regenerate. Pieces from the forward 

 part of the body regenerate heads rapidly, those from the middle por- 

 tion of the body more slowly, and those from the posterior region very 

 slowly or not at all. The readiness with which appropriate ends are 

 formed is also seen in occasional errors. A head, if severed from the 

 body, may regenerate a second head instead of a tail at its posterior 

 end. Similarly, the tail end may sometimes produce a tail instead of a 

 head at its anterior end. All of the evidence suggests that there is a 

 gradient in the worm, the head-forming tendency being strongest at the 

 anterior end and weakest at the posterior end, with a reverse gradient 



